Stimulated Raman scattering, or Raman conversion, is a process that amplifies radiation at a “Stokes” wavelength λS at the expense of radiation at a shorter “laser” wavelength λL in the presence of a Raman medium. In particular, in Raman conversion, a single photon of wavelength λS is created for each photon of wavelength λL that is annihilated in the Raman medium. The energy difference between these two photons appears as excitation of the Raman medium, whose molecules or atoms are excited by this process from a lower to a higher energy state. As the excitation is thermalized, the excitation energy is transformed into heat.
In Raman conversion, the conversion rate from λL to λS in the Raman medium is proportional to the product of the laser and Stokes radiation beam intensities (each measured, for example, in W/cm2). Therefore, the heating rate at any given point in the Raman medium is also proportional to the product of these two intensities, being small where the intensity product of the beams is small and, conversely, large where the intensity product of the beams is large. Since the heating typically is not uniform, it may cause deleterious effects such as thermal lensing of the radiation beams.